The present invention relates to the field of methods for manufacturing a multi-layered ceramic substrate used for electronic devices, and in particular to methods for manufacturing a non-shrinkable multi-layered substrate which greatly suppresses shrinkage of the substrate during firing.
Normally, multi-layered ceramic substrates are manufactured using a method called the green sheet lamination method. In this method, green sheets, made by forming a slurry containing ceramic powder and organic binder into a sheet, are punched (for holes) and screen printed with conductive paste. These green sheets are stacked to the required number, press-heated to laminate the layers, and then fired.
The advantages of this method include the feasibility of fine pattern printing realized by the extremely flexible green sheet and good permeability to organic solvents, and good surface smoothness and air-tightness which allow the lamination of even up to several dozens of layers.
On the other hand, the main disadvantage is the difficulty in achieving dimensional accuracy. This is due to shrinkage of the ceramic substrate accompanied by sintering which occurs during firing. Inaccurate dimensions cause mismatching between components and conductive patterns, generating the serious problem of inability to mount semiconductor chips such as CSPs (chip size packages) and MCMs (multi-chip modules) with high accuracy.
As a result, recent developments have been focusing on a method for eliminating lateral shrinkage during firing. This method involves the formation of shrinkage suppression sheets, using the doctor blade method, containing a ceramic material such as alumina which does not sinter at the sintering temperature of green sheet. These sheets are disposed on both faces of the green sheet laminated body and fired. The sintered multi-layered ceramic substrate then shrinks only in the thickness direction and not in the lateral direction, enabling semiconductor chips to be mounted with much higher accuracy.
FIG. 2 shows the conventional method for manufacturing a multi-layered ceramic substrate 2. After firing the multi-layered ceramic substrate 2, shrinkage suppression sheets 1 on both faces of a multi-layered ceramic substrate are removed by rotating a dry rotary brush 3 at high speed, as illustrated in FIG. 2.
However, this conventional removal method may not be able to accurately control the amount of the shrinkage suppression sheet to be removed by simply changing the rotation speed of the rotary brush or the distance to the substrate, i.e., the strength of the brush polishing the substrate. For example, too slow brush rotation speed or insufficient polishing time causes uneven removal. The conductive pattern on the surface of the multi-layered substrate may be damaged if the revolution of the brush is too fast or polishing time is too long. As a result, the conductive pattern may be disconnected or short-circuited, resulting in a low yield rate. Furthermore, in the case of an irregularly-shaped substrate with a cavity A on the surface of the multi-layered substrate, as shown in FIG. 2, residue in the cavity A is not always successfully removed by the rotary brush 3.
A method for manufacturing a multi-layered ceramic substrate of the present invention involves spraying of water, ceramic powder, or a mixture of ceramic powder and water together with compressed air for removing a shrinkage suppression sheet from a green sheet laminated body containing low-temperature firing substrate material.
The fine controllability of this method, by changing the pressure of compressed air, enables the removal of the shrinkage suppression sheet completely without causing uneven removal even if a cavity exists in the substrate. In addition, the polishing capability improves by adding ceramic powder.
Furthermore, properties of the ceramic powder used to create the green sheet laminated body remain unchanged, even if the removed shrinkage suppression sheet material mixes with the ceramic powder, because the same material is used for the ceramic powder which is sprayed and as the main constituent of the shrinkage suppression sheet. Accordingly, sprayed ceramic powder can be collected for reuse in spraying, enabling this method to be applied to circulating continuous devices.